Loading revision…
Loading revision…
Syllabus: Basic control system components; Feedback principle; Transfer function; Block diagram representation; Signal flow graph.
5 hand-picked visual resources on this topic — under each card, look for See it. Every link is checked against the live site before it ships.
Closed loop and what feedback buys you
For a forward path with feedback , negative feedback divides the open-loop gain by . The same factor divides the sensitivity of the closed-loop gain to drift in , which is the real reason feedback is used: gain is cheap, gain accuracy is not.
negative feedback; a
Where marks are lost
is the whole design lesson: precision belongs in the feedback network, not in the amplifier.
See it
Mason's gain formula
Faster than block reduction whenever loops touch each other in awkward ways, and GATE asks it directly. Two loops are non-touching if they share no node and no branch.
Block-diagram moves worth memorising
Only three rules are needed, and each is just 'keep every signal unchanged'. Moving a take-off point backwards past a block requires a in the new branch; moving it forwards requires . For a summing junction the rule is the mirror image.
sensitivity to the forward path
sensitivity to the feedback element — close to , so must be the accurate part
characteristic equation; its roots are the closed-loop poles
= gain of the -th forward path
sums over individual loops, then non-touching pairs, then triples
the determinant of what is left after deleting path
Where marks are lost
Signs: a loop gain already carries the of its own summing junction, so a unity negative-feedback loop is and . Double-counting that minus is the usual slip.
Moving it forward needs ; moving it backward needs .
superpose inputs one at a time; the denominator never changes